1. Field of the Invention
Spinal surgery methods and devices are disclosed for repairing damaged or deteriorated vertebrae at the lower lumbar levels, such as in the L5-S1 intervertebral space.
2. Description of the Relevant Art
The vertebral column is the central pillar of the body. It is a generally flexible column that bears tensile and compressive loads, permits bending motions, and provides an attachment site for ribs, muscles and other structures. The vertebral column includes irregular bones called vertebrae that are separated by fibrocartilaginous structures known as intervertebral discs. There are seven vertebral, twelve thoracic, five lumbar, five sacral, and four coccygeal vertebrae. A typical vertebra consists of a rounded anterior body and a posterior vertebral arch that together form a protective structure around the vertebral canal that contains the spinal cord.
The intervertebral discs can be damaged or undergo degeneration, which often results in painful and sometimes debilitating nerve impingement syndromes. It is sometimes necessary to surgically replace the native disc with prosthetic disc implants to relieve the pain, restore the functional mechanics of the vertebral column, and promote fusion between adjacent vertebral bodies. Procedures such as total disc arthroplasty (disc replacement) have used a direct anterior approach orthogonal to the midline of the vertebral body, but such procedures require unfettered anterior spinal exposure for precise midline placement of the prosthetic disc. The major vascular structures that run along the anterior spine must be mobilized to achieve this exposure, which typically requires the assistance of a vascular surgeon. The procedure also causes significant surgical disruption of the anterior annular element around the disc.
Bertagnoli has described an anterolateral transpsoatic approach (ALPA) for implantation of prosthetic disc replacement devices. The patient is positioned in a supine position on the operating table, with the arms in abduction. The target disc level is localized through bi-planar fluoroscopy, and an inflatable bladder is placed beneath the level of interest to permit additional lordosis. An anterolateral incision is made on the left side for access to lumbar intervertebral spaces, while the incision is made on the right side for access to L5-S1. The fascia of the external oblique muscle is opened along the direction of its fibers and the muscle is split. The retroperitoneal space is entered and the peritoneal sac mobilized away from the overlying fascia to develop an operative pathway along the anterior aspect of the psoas muscle to the lateral aspect of the intervertebral space. The target zone for annulotomy is from the one o'clock to three o'clock position above the L5-S1 level, which leaves the anterior longitudinal ligament intact and avoids mobilizing the iliac vessels. At the L5-S1 level the target annulotomy zone is from the eight o'clock to ten o'clock position with mobilization of the iliac vessel toward the midline. Injury to the left iliac vessel is an unfortunate complication of such procedures. Additional information about anterolateral approaches to spinal surgery at the L4-L5 level is found in Bertognali et al, U.S. Pat. No. 7,326,216.
A minimally invasive procedure promoted by Nuvasive, Inc. uses a direct lateral, retroperitoneal approach to access the intervertebral discs above the L5-S1 level with minimal muscular disruption. The patient is placed in a lateral decubitus position and the direct lateral incision is made in the axillary line. Another incision is made posterior to the lateral border of the erector spinae muscle, and finger dissection is conducted through this opening to the retroperitoneal space. The index finger of the surgeon sweeps the peritoneum anteriorly and palpates the psoas muscle. A dilator instrument is then introduced through the direct lateral incision and the index finger then guides the dilator instrument to the psoas muscle. The fibers of the psoas muscle are then split using blunt dissection and EMG monitoring to minimize damage to the nerves of the lumbar plexus that run through the posterior psoas muscle. A tissue distraction and tissue retraction assembly are then used to help establish an operative corridor to the direct lateral aspect of the intervertebral space at about the 3 o'clock position, as shown in U.S. Pat. No. 7,207,949. The direct lateral retroperitoneal approach to the L5-S1 space has not been possible because the anterior superior iliac spine obstructs a direct lateral approach to the L5-S1 intervertebral space. Hence approaches to the L5-S1 space typically use a standard anterior approach. For a laterally positioned patient, an extremely large sigmoidal incision has been required, with subsequent reflection of all the overlying musculature to expose the L5-S1 space.
It would therefore be useful to provide a minimally invasive approach to the L5-S1 space that minimizes injury to the blood vessels and nerves around the vertebral bodies. It would also be helpful to perform such a procedure in a manner that minimizes retroperitoneal scarring and damage to other body structures. Minimally invasive surgical approaches to the intervertebral spaces in the past have also been limited by the need to insert the prosthetic disc implant either into the front portion, posterior portion, or the side of the disc space to achieve stable placement of the prosthetic implant. It would therefore be useful to have a procedure that could avoid such a limitation at any vertebral level.